Knowledge of the chemical speciation of particulate manganese (pMn) is important for understanding the biogeochemical cycling of Mn and other particle-reactive elements. Here, we present the synchrotron-based X-ray spectroscopy-derived average oxidation state (AOS) of pMn in the surface Arctic Ocean collected during the U.S. GEOTRACES Arctic cruise (GN01) in 2015. We show that the pMn AOS is less than 2.4 when sampled during the day and more than ~3.0 when sampled at night. We hypothesize that an active light-dependent redox cycle between dissolved Mn and particulate Mn(III/IV) exists during the day-night cycle in the surface Arctic Ocean, which occurs on the timescale of hours. The magnitude of observed pMn AOS is likely determined by the net effect of the length of the previous night and integrated light level before the end of pMn sampling.

What controls the distribution of barium (Ba) in the oceans? Answers to this question have been sought since early studies revealed relationships between particulate Ba (pBa) and POC and dissolved Ba (dBa) and silicate, suggesting applications for Ba as a paleoproductivity tracer and as a tracer of modern ocean circulation. Herein, we investigated the Arctic Ocean Ba cycle through a one-of-a-kind data set containing dissolved (dBa), particulate (pBa), and stable isotope Ba (δ138Ba) data from four Arctic GEOTRACES expeditions conducted in 2015. We hypothesized that margins would be a substantial source of Ba to the Arctic Ocean water column. The dBa, pBa, and δ138Ba distributions all suggest significant modification of inflowing Pacific seawater over the shelves, and the dBa mass balance implies that ~50% of the dBa inventory (upper 500 m of the Arctic water column) is not supplied by conservatively advected inputs. Calculated areal dBa fluxes are up to 10 µmol m-2 d-1 on the margin, which is comparable to fluxes described in other regions. Applying this approach to dBa data from the 1994 Arctic Ocean Survey yields similar results. Surprisingly, the Canadian Arctic Archipelago did not appear to have a similar margin source; rather, the dBa distribution in this section is consistent with mixing of Arctic Ocean-derived waters and Baffin-bay derived waters. Although we lack enough information to identify the specifics of the shelf sediment Ba source, we suspect that a terrigenous source (e.g., submarine groundwater discharge or fluvial particles) is an important contributor

Particle composition is an important parameter that influences sinking velocity of marine particles. Most current studies, however, are limited by either a lack of routine measurements of particle composition or low sampling resolution in the water column. Here, we compile full ocean-depth size-fractionated (1-51 and >51 μm) particle concentration and composition of suspended particulate matter from three recent U.S. GEOTRACES cruises to calculate their corresponding sinking velocity and mass flux. Our model is based on Stokes’ Law and incorporates a newly updated power-law relationship between particle size and porosity. The integration of the porosity-size relationship decreases the power applied to size in Stokes’ Law to 0.8. The medians of average sinking velocity in total particles are 15.4, 15.2, and 7.4 m/d, in the North Atlantic, Southeast Pacific, and western Arctic Ocean, respectively. We examine the relative importance of particle concentration, composition, size, and hydrography on sinking fluxes. Particle concentration is the major control of the variability and magnitude of mass flux, while particle composition is the second most important term. Increasing porosity with aggregate size and a dominance of smaller particles diminishes the importance of the size dependence in mass flux, elevating the relative importance of composition and thus density. Viscosity of seawater can result in up to a factor of two difference in mass flux between polar and tropical oceans. This work serves as one of the first studies to offer quantitative perspectives for the contribution from different factors to mass flux in field observations of marine particles.

In order to better understand the sources, sinks and hydrodynamic/biogeochemical influences on particulate matter distribution and variability in Arctic basins, we combined data from two 2015 fall expeditions: one from Bering Strait (USCGC Healy) and the other from Barents Sea (R/V Polarstern) meeting at the North Pole. Sections of beam attenuation due to particles were overlain by salinity, temperature, and chlorophyll-a fluorescence (Chl-a), and with nitrate contours on Chl-a sections to compare with concentrations of particulate matter (PM) and particulate organic carbon (POC) from full water column filtered samples. Dense Pacific water moving swiftly through Bering Strait erodes and carries sediment-laden waters onto the Chukchi Shelf, much of it moving in and above Barrow Canyon or is entrained in eddies. This nutrient-rich Pacific water sinks below the low-salinity, nutrient-poor polar mixed layer, forming a thick lens of high salinity water known as Pacific halocline waters. The nutrient-poor mixed layer inhibits photosynthesis in surface waters of Canada and Makarov Basins, but subsurface Chl-a maxima are observed when nutrients are available. Surface-water POC biomass appears greater in Barents Sea than in Beaufort Sea because nutrient-rich Atlantic water entering Barents Sea is not isolated from surface waters by strong stratification. Surface water freezes, creating high-density water that cascades into 400 m basins in Barents Sea and into deep Nansen Basin, eroding sediment that forms patches of nepheloid layers in the shallow basins. Nepheloid layers in the deep basins are very weak, consistent with a lack of strong currents there.